Process for the recovery of sulphur from a hydrogen sulphide, containing gas

ABSTRACT

The invention is directed to a process for the recovery of sulphur from a hydrogen sulphide containing gas, which process comprises; i) oxidizing part of the hydrogen sulphide in a gaseous stream with oxygen or an oxygen containing gas in an oxidation stage to sulphur dioxide; ii) reacting the product gas of this oxidation stage in at least two catalytic stages, in accordance with the Claus equation: 2 H 2 S+SO 2 →2 H 2 O+3/n S n ; iii) catalytically reducing SO 2  in the gas leaving the last of said at least two catalytic stages, wherein the catalytic reduction takes place in a catalyst bed downstream from the last Claus catalytic stage.

BACKGROUND OF THE INVENTION

In a number of processes, such as the refining of crude oil, the purification of natural gas and the production of synthesis gas from, for example, fossil fuels, sulphur containing gas, in particular H₂S containing gas, is released. On account of its high toxicity and its smell, the emission of H₂S is not permissible.

The best known and most suitable process for removing sulphur from gas by recovering sulphur from hydrogen sulphide is the so-called Claus process. In this process hydrogen sulphide is converted by oxidation to a considerable extent into elemental sulphur; the sulphur thus obtained is separated from the gas by condensation. The residual gas stream (the so-called Claus residual gas) still contains some H₂S and SO₂.

The method of recovering sulphur from sulphur containing gases by the so-called Claus process is based on the following overall reactions:

2 H₂S+3 O₂→2 H₂O+2 SO₂  (1)

4 H₂S+2 SO₂⇄4 H₂O,+6/n S_(n)  (2)

Reactions (1) and (2) result in the main reaction:

2 H₂S+O₂⇄2 H₂O+2/n S_(n)  (3)

A conventional Claus converter—suitable for processing gases having an H₂S content of between 50 and 100%, comprises a burner with a combustion chamber, the so-called thermal stage, followed by a number of reactors generally two or three—filled with a catalyst. These last stages constitute the so-called catalytic stages.

In the combustion chamber, the incoming gas stream, which is rich in H₂S, is combusted with an amount of air at a temperature of approximately 1200° C. The amount of air is adjusted so that one third of the H₂S is fully combusted to form SO₂ in accordance with the following reaction:

 2 H₂S+3 O₂→2 H₂O+2 SO₂  (1)

After this partial oxidation of H₂S the non-oxidised part of the H₂S (i.e. basically two-thirds of the amount offered) and the SO₂ formed react further as to a considerable portion, in accordance with the Claus reaction

4 H₂S+2 SO₂⇄4 H₂O+3 S₂  (2)

Thus, in the thermal stage, approximately 60% of the H₂S is converted into elemental sulphur.

The gases coming from the combustion chamber are cooled to about 160° C. in a sulphur condenser, in which the sulphur formed is condensed, which subsequently flows into a sulphur pit through a siphon.

The non-condensed gases, in which the molar ratio of H₂S:SO₂ is unchanged and still 2:1, are subsequently heated to about 250° C., and passed through a first catalytic reactor in which the equilibrium

4 H₂S+2 SO₂⇄4 H₂O+6/n S_(n)  (2)

is established.

The gases coming from this catalytic reactor are subsequently cooled again in a sulphur condenser, in which the liquid sulphur formed is recovered and the remaining gases, after being re-heated, are passed to a second catalytic reactor.

When the gaseous feedstock contains H₂S concentrations of between about 15 and 50%, the above described “straight through” process is not used, but instead a variant thereof, the so-called “split-flow” process. In the latter process one-third of the total amount of feedstock is passed to the thermal stage and combusted completely to SO₂ therein. Two-thirds of the feedstock is passed directly to the first catalytic reactor, by-passing the thermal stage. When the feedstock contains H₂S concentrations of between 0 and 15 the Claus process can no longer be used. The process then used is, for example, the so-called Recycle SELECTOX process, in which the feedstock is passed with an adjusted amount of air into an oxidation reactor, the so-called oxidation reactor, the so-called oxidation stage. The reactor contains a catalyst which promotes the oxidation of H₂S to SO₂, and the amount of oxidation air is adjusted so that an H₂S:SO₂ ratio of 2:1 is established, whereafter the Claus reaction proceeds. The gas from the oxidation reactor is cooled in a sulphur condenser, in which the sulphur formed is condensed and discharged.

To dissipate the reaction heat generated in the oxidation reactor, a portion of the gas stream coming from the sulphur condenser is recirculated to the oxidation reactor.

It is clear that in the Recycle SELECTOX process, the oxidation stage, which is catalytic and does not lead to high temperatures, is equivalent to the thermal stage in the Claus process. In the following, both the thermal Claus stage and the oxidation stage of the Recycle SELECTOX process are referred to as oxidation stages.

The sulphur recovery percentage in a conventional Claus converter is 92-97%, depending on the number of catalytic stages

By known processes, the H₂S present in the residual gas from the Claus reaction is converted, by combustion or some other form of oxidation, into SO₂ whereafter this SO₂ is emitted to the atmosphere. This has been permissible for low concentrations or small amounts of emitted SO₂ for a long time. Although SO₂ is much less harmful and dangerous than H₂S this substance is also so harmful that its emission is also limited by ever stricter environmental legislation.

As has been observed, in the Claus process as described above, in view of the equilibrium reaction which occurs, the H₂S:SO₂ ratio plays an important role. In order to obtain an optimum conversion to sulphur, this ratio should be 2:1. Generally speaking, this ratio is controlled by means of a so-called H₂S/SO₂ residual gas analyser. This analyser measures the H₂S and SO₂ concentrations in the residual gas. A controller then maintains the ratio of 2:1 constant on the basis of the equation

 [H₂S]−2[SO₂]=0

by varying the amount of combustion air, depending on the fluctuations in the gas composition and the resulting deviation in the above equation. Such a control of the process, however, is highly sensitive to these fluctuations.

Furthermore, the sulphur recovery efficiency (calculated on the amount of H₂S supplied) is no higher than 97%, and so the gas flowing from the last catalytic stage the residual gas—still contains substantial amounts of H₂S and SO₂, determined by the Claus equilibrium, and this in a molar ratio of 2:1.

The amount of H₂S present in the residual gas can be separated by absorption in a liquid.

The presence of SO₂ in the residual gas, however, is a disturbing factor during the further processing thereof and must therefore be removed prior to such further processing. This removal and hence the after-treatment of the gas is complicated.

The great disadvantage of the presence of SO₂ is that this gas reacts with conventional liquid absorbents to form undesirable products. To prevent undesirable reactions of the SO₂, therefore, the SO₂ is generally catalytically reduced with hydrogen to form H₂S over an Al₂O₃ supported cobalt-molybdenum catalyst in accordance with the so-called SCOT process. The total amount of H₂S is subsequently separated by liquid absorption in the usual manner.

In the SCOT process the sulphur components, other than H₂S, such as SO₂ (sulphur dioxide) and sulphur vapour (S₆ and S₈) are fully hydrogenated to H₂S according to the following reactions:

SO₂+3 H₂→H₂S+2 H₂O  (4)

S₆+6 H₂→6 H₂S  (5)

S₈+8 H₂→8 H₂S  (6)

Other components, such as CO, COS and CS₂, are hydrolysed according to:

 COS+H₂O→H₂S+CO₂  (7)

CS₂+2 H₂O→2 H₂S+CO₂  (8)

CO+H₂O→H₂+CO₂  (9)

Above conversions to H₂S are performed with a cobalt-molybdenum catalyst on alumina at a temperature of about 280-330° C. For the SCOT process it is required that sulphur vapour is hydrogenated to H₂S, and also that SO₂ is completely converted to H₂S down to ppm level, to prevent plugging/corrosion in the down-stream water quench column. This type of hydrogenation can be defined as high temperature hydrogenation.

In accordance with another method, for example, the BSR SELECTOX process, after reduction of the SO₂ in residual gas to H₂S and after condensation of the water vapour, the gas is passed into an oxidation reactor, as in the Recycle SELECTOX process. The oxidation air is adjusted so that an H₂S:SO₂ ratio of 2:1 is obtained, whereafter the Claus reaction proceeds. Both in the SCOT process and in the BSR SELECTOX process, the removal of SO₂ from the residual gas is a relatively expensive operation.

The above described after-treatment of the gases, carried out by means of a so-called tail gas treater, which involves an investment of another 50-100% of the cost of the preceding Claus converter, can result in an increase of the sulphur recovery efficiency of up to 98-99.8%.

In U.S. Pat. No. 4,988,494, it is proposed that the H₂S concentration in the gas leaving the last catalytic Claus stage is controlled to have a value ranging between 0.8 and 3% by volume by reducing the quantity of combustion or oxidation air passed to the oxidation stage.

The increase of the H₂S concentration will result in a decreased SO₂ concentration, however, not to very low levels. For an H₂S concentration of 0.8% by volume, the SO₂ concentration will be typically 0.03-0.15% by volume, and this will result in a sulphur recovery efficiency loss of typically 0.09-0.45%.

In the process according to this patent, the H₂S is selectively oxidised in a dry bed oxidation stage.

As SO₂ is not converted in a dry-bed oxidation stage, this will result in appreciable sulphur recovery losses, and consequently sulphur recovery efficiencies close to 100% can not be reached.

A second disadvantage of operating with excess H₂S compared to SO₂ is that the temperature increase in the drybed oxidation reactor becomes higher with increasing H₂S concentration.

Higher reactor temperatures will result in an increased formation of SO₂ as a result of gas-phase and catalytic oxidation of formed sulphur vapour. Also for this reason, a shifted operation towards H₂S of the Claus converter is not beneficial.

It has been experienced, that in case the catalyst bottom temperature in a dry-bed oxidation reactor exceeds 250-260° C., the oxidation efficiency to elemental sulphur will start to drop from 94-96% to lower values. Combined with a reactor inlet temperature of approximately 180-200° C., this results in a temperature increase of some 60-80° C., corresponding with 1.0-1.2 vol. % of H₂S in the process gas.

The shifted operated sulphur plant, followed by a drybed oxidation step with an oxidation catalyst which is not effective in promoting the Claus reaction, is known as the SUPERCLAUS™ or SUPERCLAUS™-99 process.

The SUPERCLAUS™ process, as well as the SUPERCLAUS™-99.5 process, is described in -SUPERCLAUS™-the answer to Claus plant limitations″, Lagas, J. A.; Borsboom, J; Berben, P. H., 38^(th) Canadian Chemical Engineering Conference, Edmonton, Canada.

It is known, that removal of SO₂ from a process gas can be performed according to a high temperature hydrogenation step to H₂S as applied in the SCOT process or in the SUPERCLAUS™-99.5 process.

In European patent No. 669,854 the selective hydrogenation of SO₂ to elemental sulphur has been described. It has been mentioned that this hydrogenation could suitably be applied downstream of a regular Claus unit, prior to dry bed oxidation.

However, the specific process conditions required according to this patent are not easily compatible with the composition of gas coming from a Claus unit. This means that rather complicated, and thus costly, measures are necessary to satisfy these conditions.

BRIEF DESCRIPTION OF THE INVENTION

Surprisingly it has now been found that it is possible to increase the sulphur recovery by very simple process modifications.

According to the invention a process for the recovery of sulphur from a hydrogen sulphide containing gas comprises:

i) oxidising part of the hydrogen sulphide in a gaseous stream with oxygen or an oxygen containing gas in an oxidation stage to sulphur dioxide;

ii) reacting the product gas of this oxidation stage in at least two catalytic stages, in accordance with the Claus equation

2 H₂S+SO₂→2 H₂O+3/n S_(n)

iii) catalytically reducing SO₂ in the gas leaving the last of said at least two catalytic stages, wherein the catalytic reduction takes place in a catalyst bed downstream from the last Claus catalytic stage.

One aspect of the invention is the reduction of SO₂. This reduction of SO₂ to elemental sulphur, to H₂S or to a mixture of both, by hydrogenation, requires the presence of hydrogen. In the gas leaving the last catalytic Claus stage sufficient hydrogen is normally present. This hydrogen is produced in the thermal stage, by, among others, cracking of H₂S to hydrogen and sulphur vapour. In case the amount of hydrogen is insufficient, additional hydrogen may be added to the gas, by adding a stream of concentrated hydrogen, or by generating hydrogen by sub-stoichiometric combustion of fuel gas in in-line process heaters.

Also CO is usually present in the gas containing SO₂. Because of the reducing properties of CO, this component is capable of reducing SO₂. In this way CO acts in the same way as hydrogen, and a mixture of hydrogen and CO is therefor also suitable for reducing SO₂.

CO+H₂O→CO₂+H₂

SO₂+2 CO→2CO₂+1/n S_(n)

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a feedstock Claus gas process according to the present invention;

FIG. 2 is a schematic diagram of lean feedstock Claus gas process of the invention;

FIG. 3 is a scheratic diagram of the oxidation or absorption process in a dry bed and the oxidation in a liquid in detail during the sulfur removal stage after the residual gas is supplied; and

FIG. 4 is a schematic diagram of another feedstock Claus gas process, similar to FIG. 1, but includes a separate hydrogenation stage.

DETAILED DESCRIPTION OF THE INVENTION

In the context of this invention the terms ‘reduction’ and ‘hydrogenation’ have both been used to indicate the same.

It was found that reduction of SO₂ to H₂S proceeds in two steps. The first step is the hydrogenation of SO₂ to elemental sulphur vapour, according to:

SO₂+2 H₂→1/n S_(n)+2 H₂O  (10)

The temperature range of this reaction is typically 130-230° C. Complete conversion of SO₂ is achieved at approximately 200-230° C. over a conventional cobalt-molybdenum-catalyst.

The hydrogenation of sulphur vapour to H₂S starts at approximately 220° C., and is completed at approximately 260 300° C. All temperature levels depend also on space velocity, H₂O and SO₂ concentration, H₂/SO₂ ratio and type of catalyst.

A suitable temperature range to convert SO₂ is typically above 200° C., whereas a suitable upper limit is 225° C. For an acceptable conversion of SO₂, the catalyst temperature should be high enough to stay above the sulphur vapour dewpoint. The presence of oxygen in the gas to be hydrogenated is not harmful to the catalyst or catalyst performance, as oxygen is hydrogenated to water under approximately the same reaction conditions as apply to SO₂.

The hydrogenation catalyst can be selected from the group of metals of group VI and VIII of the periodic table of elements. The catalyst may be supported and is preferably in the sulphidic form. Examples are iron, nickel and cobalt for Group VIII, and molybdenum, tungsten and chromium for Group VI. It is preferred to use cobalt-molybdenum or nickel-molybdenum.

In case the gas coming from the SO₂ hydrogenation step is not further subjected to a treatment, it has advantages to, adjust the molar ratio of H₂S to SO₂ after the final Claus stage, to a value different from 2:1, i.e. a higher amount of SO₂, which results in a high sulphur recovery, with only very limited efforts. Generally it is preferred to have a ratio of from 1:1 to 1:10.

It should be noted, that complete conversion of all sulphur compounds to H₂S down to the ppmv-level as in the SCOT process, is not required. Hydrogenation of SO₂ down to a level of approximately 100 ppmv is acceptable and will result in negligible recovery efficiency losses.

It is not very critical whether the reduction of SO₂ proceeds to elemental sulphur or to H₂S. Generally a mixture of both will be obtained, which has preference.

In the SUPERCLAUS™ process, as described in U.S. Pat. No. 4,988,494, a residual concentration of SO₂ is left in the process gas to the dry-bed oxidation stage. This residual SO₂ not only decreases the overall sulphur recovery efficiency, because it is not converted to elemental sulphur, but also decreases the activity of the selective oxidation catalyst.

To overcome this decrease of activity, the temperature level in the dry-bed oxidation reactor has to be increased, but a higher temperature level will decrease the oxidation efficiency to elemental sulphur. Consequently, very high oxidation efficiencies in the dry-bed oxidation stage, in the range of 94-96%, are not possible with process gas containing concentrations of SO₂ of more than about 250 ppmv.

It is one of the advantages of the present invention, that it allows the Claus process to be operated with a residual concentration of SO₂ which would otherwise be prohibitive for a downstream dry bed oxidation process.

The effect of a deactivated Claus catalyst, which results in an increased amount of SO₂ in the gas coming from the Claus unit, has the same negative effects on the overall sulphur recovery efficiency in case of a final dry-bed oxidation stage, because the SO₂ is not converted in the drybed oxidation stage.

Incorporation of a hydrogenation step, as in the present invention, makes the process not sensitive anymore to Claus catalyst deactivation, which is a major advantage. In this way the reduction step also acts as a safeguard and serves to maintain a constant and high sulphur recovery.

The control of the process with an incorporated hydrogenation step will also be much more flexible. A varying SO₂ content in the process gas from the last catalytic Claus reactor will not result in varying losses by SO₂ slippage, because all SO₂ is converted. This makes the control on H₂S much less sensitive to process fluctuations.

According to a further embodiment the process comprises an additional step of selectively oxidising the hydrogen sulphide in the product gas of step iii) to elemental sulphur.

In this way of operation of the process it is not necessary to modify the ratio of H₂S to SO₂, as in some of the prior art processes. Accordingly the ratio may be kept at levels close to the Claus equilibrium level of 2:1. A suitable range for this is from 10:1 to 1:2.

In detail the process of the invention can be described as comprising:

i) oxidising part of the hydrogen sulphide in a gaseous stream with oxygen or an oxygen containing gas in an oxidation stage to sulphur dioxide;

ii) reacting the product gas of this oxidation stage in at least two catalytic stages, in accordance with the Claus equation:

2 H₂S+SO₂⇄2 H₂O+3/n S_(n)

iii) catalytically reducing SO₂ in the gas leaving the last of said at least two catalytic stages, preferably employing for this purpose a reduction catalyst or a catalytic stage including a reduction catalyst which is effective to promote reduction of SO₂

iv) selectively oxidising H₂S in the gas leaving the hydrogenation to elemental sulphur, preferably employing for this purpose a catalytic stage including a selective oxidation catalyst which is substantially insensitive to the presence of water vapour in the gas stream, is ineffective in promoting establishment of the equilibrium

2 H₂S+SO₂⇄2 H₂O+3/n S_(n), and

 is effective to promote oxidation of HS to sulphur in the presence of water vapour.

It is preferred in said step iv) of selectively oxidising H₂S also employing a stoichiometric excess of oxygen sufficient to result in an overall excess of oxygen being employed in the total process for the recovery of sulphur from the hydrogen sulphide-containing gas.

In the process according to the present invention, the concentration of the hydrogen sulphide gas in the residual gas can be controlled in a simple way. Thus, for example, the signal from an H₂S analyser in the residual gas can be used to set or adjust the amount of combustion air or oxidation air supplied to the oxidation stage.

The reduction step can be incorporated in a commercial process in several ways. The most economic way is to install a layer of hydrogenation catalyst below a layer of Claus catalyst, in the last catalytic Claus reactor, usually the second or third Claus reactor. The process conditions should fit for the hydrogenation conditions, such as temperature, sulphur dewpoint margin, sufficient hydrogen available. Usually this will be the case.

In another embodiment, the hydrogenation step is carried out in a separate (small) hydrogenation reactor, with or without prior removal of elemental sulphur from the gas to be hydrogenated.

Especially in case the gas contains substantial amounts of CO, it is advantageous to carry out the reduction under conditions that most of the sulphur vapour has been removed. In the presence of a hydrogenation (reduction) catalyst, CO and sulphur react, thereby producing unwanted by-products, such as COS, resulting in a decrease of sulphur recovery. In these cases it is preferred that the gas coming from the last Claus reactor is first subjected to a sulphur condensation step, with removal of condensed sulphur, followed by the reduction step of the present invention, optionally after reheating.

After the reduction step the gas is then further treated, such as in an oxidation stage to convert H₂S to elemental sulphur.

An alternative process route, without a final dry-bed oxidation stage, may be attractive, with lower investment costs and somewhat lower sulphur recovery efficiency. This process alternative consists of a Claus converter with an oxidation stage and at least two catalytic Claus stages, the H₂S concentration in the gas leaving the last of said at least two catalytic stages being controlled to have a value ranging between 0.1 and 0.4% by volume by increasing the quantity of combustion or oxidation air passed to the oxidation stage followed by reducing SO₂ in the gas leaving the last of said at least two catalytic stages in a hydrogenation step.

Depending on the number of catalytic Claus stages, a recovery efficiency of 98.0-99.5% can be reached.

In the context of this application, the term ‘catalytic Claus stage’ refers to the actual reactor containing the Claus catalyst. Depending on the plant set-up, the usual additional equipment may be present, such as reheaters or sulphur condensers.

The hydrogenation can also be applied down-stream of the dry-bed oxidation stage (SUPERCLAUS™ reactor) , to convert the formed SO₂. This will increase the overall performance of the SUPERCLAUS™ process significantly.

In order to unload tail gas treating units such as SCOT, it is beneficial to install a hydrogenation stage down-stream of the last Claus catalytic stage. This will reduce the amount of sulphur components to be hydrogenated in the SCOT unit significantly, and will reduce solvent circulation and stripping-steam requirements.

The process according to the invention can be suitably applied for the treatment of gases containing hydrogen sulphide, but also for gases containing both hydrogen sulphide and substantial quantities of ammonia (cf NL-C 176160), in the latter case, the temperature in the Claus combustion chamber should be at least 1250° C.

A disadvantage of operating with excess H₂S compared to SO₂ in the Claus tail gas is, that this operation mode results in less combustion air to the main (H₂S) burner compared to the conventional mode of operation with H₂S:SO₂=2:1. This will result in a decrease of temperature of combustion in the combustion chamber, which is detrimental to the destruction efficiency of ammonia, which requires high temperatures.

In the process according to the invention, the hydrogen sulphide gas remaining in the residual gas can be processed to form sulphur by a known per se method. Such methods are described in the literature. Preferably, however, the remaining gaseous hydrogen sulphide is oxidized with air in an oxidation stage to form elemental sulphur in accordance with the following reaction:

2 H₂S+O₂→2 H₂O+2/n S_(n)  (3)

It was found that, when the concentration of the hydrogen sulphide leaving the last catalytic hydrogenation stage is maintained at a value of between 0.5 and 1.2% by volume, after this selective oxidation, an optimum sulphur recovery percentage of 99.6-99.8 can be obtained. The oxidation can in principle take place in two ways, namely, by dry-bed oxidation or by oxidation in a liquid, in which, in general, sulphur and water vapour have first been removed from the residual gas.

In the dry-bed oxidation, the H₂S concentration in the residual gas is preferably maintained between 0.5 and 1.2 by volume, because above 1.2% by volume of H₂S the total sulphur recovery percentage is decreased.

In the dry oxidation bed, the oxidation to sulphur can be effected by a known per se method using an oxidation catalyst. One example of an oxidation catalyst and the application thereof is described in U.S. Pat. No. 4,311,683.

The method described therein is the SELECTOX process (Hass, R. H.; Ingalis, M. N.; Trinker, T. A.; Goar, B. G.; Purgason, R. S. S.: “Process meets sulphur recovery needs”, Hydrocarbon Processing, May 1981, pages 104-107). In this process, H₂S is oxidised to 5 and SO₂ using a special catalyst. Approximately 80% of the H₂S supplied is oxidised to elemental sulphur, if water vapour is removed to a substantial extent. Another application of a dry-bed process which is not sensitive to water vapour in the process gas is the absorption of H₂S in an absorption mass as described, the example, in European patent no. 71983.

In accordance with a particularly preferred embodiment of the process according to the present invention, the oxidation is carried out in a dry bed, using a catalyst, such as described in U.S. Pat. Nos. 4,818,740 and 5,286,697 or in WO-A 9732813, the contents of which is incorporated herein by way of reference.

The use of these catalysts for the oxidation in a dry bed of the hydrogen sulphide containing residual gases obtained using the process according to the present invention has the important, in particular economic advantage that such a catalyst is practically insensitive to the presence of water vapour in the residual gas, so that the removal of sulphur and water vapour from this residual gas is unnecessary, unless it is required for other reasons, such as described herein before.

The minimum volume percent of H₂S, corresponding with the maximum volume percent of SO₂, in the residual gas from the last Claus stage, is determined by the ratio H₂/SO₂ in the gas, which should be higher than two for acceptable SO₂ hydrogenation. For this reason, the H₂S volume percent cannot be lowered too much. Also, when the H₂S volume percentage to the dry-bed oxidation reactor is too low, the oxidation efficiency to sulphur in the dry-bed oxidation reactor is not at its maximum.

When the H₂S volume percentage is too high, the overall recovery efficiency will also decrease. For above reasons, the optimum H₂S volume percentage in the process gas from the hydrogenation stage, is preferably in the range of 0.5-1.2 Vol.%.

The liquid oxidation, too, can take place using a known process. Examples of known processes are the STRETFORD process (the Chemical Engineer, February 1984, pages 84 ff), the LO-CAT process of Air Resources Inc. or the TAKAHAX process.

The control of the oxidation air to the selective oxidation is not critical and can thus be kept simple.

The process according to the invention can be carried out in an existing Claus plant and requires only relatively simple modifications of the existing control of the gas streams. In case a 2 stage Claus plant including a hydrogenation step is used, a selective oxidation reactor will have to be provided in this specific embodiment of the present invention, which in relation to the cost involved in other residual gas processing plants is inexpensive. Thus the application of the process according to the present invention leads to considerable economic advantages.

In case a 3 stage Claus plant is used only the third catalytic reactor needs to be arranged as a selective oxidation reactor and the second Claus reactor has to be provided with a layer of a reduction catalyst. In this case, too therefore, a considerable economic benefit is obtained.

The elemental sulphur that is produced in the system can be condensed from the gas flows using conventional systems, such as condensers. In case an especially high sulphur recovery is necessary it may be advantageous to use the system disclosed in EP-A 655,414, more in particular for the treatment of the final product gas flow, after the last treatment.

The process according to the present invention will now be described in more detail with reference to the accompanying FIGS. 1, 2 and 3.

As shown in FIG. 1, the feedstock gas (=Claus gas) is supplied through line 1 to the Claus burner with combustion chamber 2. The amount of combustion air, controlled by the quantity-proportion regulator 3 and H,S analyser 23, is supplied to Claus burner 2 through line 4. The heat generated during the combustion (1200° C.) of the Claus gas is dissipated in a boiler heated by spent gases, producing steam that is discharged through line 6.

The Claus reaction takes place in the burner and the combustion chamber. The sulphur formed is condensed in boiler 5 (150° C.) and discharged through line 7. The gas is passed through line 8 to a heater 9 where it is heated to the desired reaction temperature of 250° C. before being supplied through line 10 to the first Claus reactor 11. In reactor 11 the Claus reaction takes place again, whereby sulphur is formed. The gas is discharged through line 12 to the sulphur condenser 13. The condensed sulphur (150° C.) is discharged through line 14. Thereafter the gas is passed through line 15 to the next reactor stage, which again includes a heater 16, a reactor 17 and a sulphur condenser 18. In this reactor the Claus reaction takes place again in the top-part of the reactor on Claus catalyst layer 32. In the bottom part a layer 33 of a reduction catalyst is installed, where the residual SO₂ is hydrogenated. The condensed sulphur (150° C.) is discharged through line 19. The steam generated in the sulphur condensers is discharged through lines 20 and 21.

The H₂S concentration in the residual gas line 22 is controlled by an H₂S analyser 23 to a range of from 0.5-1.2% by volume. The H₂S analyser controls a control valve in combustion air line 24.

The residual gas is passed through line 22 to the sulphur removing stage 26. This sulphur removing stage may be a known sulphur removing process, such as, for example, a dry-bed oxidation stage, an absorption stage or a liquid oxidation stage. The air required for the oxidation is supplied through line 27. The sulphur formed is discharged through line 28.

The gas is then passed through line 29 to an afterburner 30 before being discharged through chimney 31.

As shown in FIG. 2, a lean Claus feedstock gas is supplied through line 1 to an oxidation reactor 2. An amount of oxidation air controlled by the quantity ratio regulator 3 and H₂S analyser 19 is passed to the oxidation reactor through line 4. In the oxidation reactor a portion of the H₂S is oxidised over a special catalyst to form SO₂ whereafter the Claus reaction takes place.

To prevent an unduly high temperature from arising within the oxidation reactor, as a result of the reaction heat, a quantity of gas is recycled by means of blower 6 through lines 5 and 7. The gas from reactor 2 is passed through line 8 to sulphur condenser 9, where the sulphur formed during the reaction is condensed at 150° C. and discharged through line 10. The heat generated during the reaction is dissipated in sulphur condenser 9 with generation of steam, which is discharged through line 11. The gas is passed through line 12 to a heater 13, where it is heated, for example, to 220° C. before being supplied to the Claus reactor 14. In reactor 14, the Claus reaction takes place again in catalyst layer 29 in the top part of the reactor. In the bottom part of reactor 14, SO₂ in the process gas is hydrogenated by layer 30 consisting of a reduction catalyst. In condenser 15, the sulphur is condensed at 150° C. and discharged through line 16, and steam generated is discharged through line 17. The H₂S concentration in the residual gas line 18 is controlled by an H₂S analyser 19 to a range of from 0.5-1.2% by volume. The H₂S analyser controls a control valve in the combustion air line 20.

The residual gas is passed through line 18 to the sulphur removing stage 22. The air required for the oxidation is supplied through line 23. The sulphur formed is discharged through line 24. The gas is then passed through line 25 to an after-burner 26 before being discharged through chimney 27.

FIG. 3 shows in greater detail the oxidation or absorption in a dry bed and the oxidation in a liquid as indicated more generally in 26 of FIG. 1 or 22 of FIG. 2. In FIGS. 1 and 2, the residual gas is supplied through lines 22 and 18, respectively, in FIG. 3, the residual gas is supplied through line 1.

In FIG. 3a, after the removal of the sulphur from the residual gas in separator 2, which is discharged through line 3, and the condensation of the water in 4, which is discharged through line 5, the gas is supplied through a heater 6 to an oxidation reactor 7. The removal of sulphur and water in 2 and 4 respectively, can take place using a known method, for example, as disclosed in U.S. Pat. No 4,526,590. In the oxidation reactor 7, a catalyst may be provided, for example, as described in the French patent publications 8009126, 8105029 or 8301426. The required oxidation air is supplied through line 8.

From the reactor, the gas flows to a sulphur condenser 9. The sulphur condensed is discharged through line 10, and the steam generated through line 11. The gas next flows through line 12 to the after-burner as designated by 30 in FIG. 1 and 26 in FIG. 2.

As shown in FIG. 3b, the residual gas is supplied through line 1 and heater 2 direct to the oxidation stage, that is to say, without a preceding sulphur and water removing stage. This embodiment can be used when a catalyst is present in the oxidation reactor 4, as described above, consisting of a non-Claus active carrier to which at least 0.1% by weight of a catalytically active material, in particular a metal oxide, has been applied, so that the specific area of the catalyst is more than 20 m²/g, while the average pore radius is at least 25 Å. The oxidation air required is supplied through line 3. The sulphur condensed in sulphur condenser 5 is discharged through line 6 and the steam generated through line 7. The gas next flows through line 8 to the after burner designated by 30,in FIG. 1, and by 26 in FIG. 2.

As shown in FIG. 3c the residual gas is passed through line 1 to a reactor 2 filled with an absorption mass, for example, as described in European patent no. 71983. In reactor 2, the hydrogen sulphide is removed from the residual gas by absorption. The gas next flows through line 3 to the afterburner, designated by 30 in FIG. 1 and by 26 in FIG. 2. When the bed is saturated, it is regenerated. Reactor 4 is connected in parallel to reactor 2 and is regenerated. By means of a circulation blower 5, a quantity of gas is circulated. This gas is heated in heater 6. The air required for the oxidation is supplied through line 7. The gas flows from reactor 4 to sulphur condenser 8. The sulphur condensed is discharged through line 9 and the steam generated through line 10. To keep the system at the required pressure a small gas stream is discharged through line 11 and recycled to the feedstock for the Claus plant (line 1 in FIGS. 1 and 2).

As shown in FIG. 3d, sulphur is removed in separator 2, which is discharged through line 3. Subsequently, in condenser 4, water is condensed which is removed through line 5. The gas is passed to a liquid oxidation stages 6. The oxidation stage may contain, for example, a basic solution of sodium carbonate, ADA (anthraquinone disulphonic acid) and sodium metavanadate, as used in the well-known STRETFORD process.

H₂S is absorbed in the liquid and subsequently oxidised with air. The oxidation air is supplied through line 7 and the sulphur formed is discharged through line 8. The gas next flows through line 9 to the after-burner (30 in FIG. 1 and 26 in FIG. 2). FIG. 4 shows an installation which is similar to that of FIG. 1, with the exception of the drybed oxidation stage, and including a separate hydrogenation stage. Starting with the third catalytic Claus stage comprising reheater 34, reactor 35 and sulphur condenser 36, the process gas is routed via line 39 to reheater 40. Reactor 41 contains a catalyst active for the hydrogenation of SO₂. Analyser 23 in line 39 controls the content of H₂S in the process gas.

The invention is illustrated in and by the following examples.

EXAMPLE 1

Using the apparatus as described with reference to FIGS. 1 and 3a, the Claus reaction is performed in a Claus plant having two catalytic Claus stages and a hydrogenation stage. Supplied to the thermal stage are a Claus gas, containing 90% by volume of H₂S, corresponding to 90 kmoles/h, 4% Vol. CO₂, 5 Vol. H₂O and 1% Vol. C₂H₆, and 48.5 kmoles/h O₂ (a “deficit” of 0%) as air oxygen. The H₂S volume percentage in the residual gas after the catalytic hydrogenation stage is 0.6, and the SO₂ content therein negligible. After the removal of sulphur and water and using a dry-bed process for the oxidation of H₂S with an oxidation efficiency of 80%, a total sulphur recovery percentage of 99.5 is obtained.

EXAMPLE 2

Using the apparatus as described with reference to FIGS. 1 and 3b, the Claus reaction is performed in a Claus plant having two catalytic stages and a hydrogenation stage. Supplied to the thermal stage are a Claus gas containing 90% by volume of H₂S corresponding to 90 kmoles/h, 4% by volume of CO₂, 5% by volume of H₂O and 1% by volume of C₂H₆, and 48.5 kmoles/h of O₂ (a “deficit” of 0%) as air oxygen. The H₂S volume percentage in the residual gas after the catalytic hydrogenation stage is 0.57, the SO₂ content therein is immeasurably small, and its water content is 34.3 by volume. The dry-bed oxidation is carried out using a water insensitive oxidation catalyst, comprising a silica carrier (Degussa OX-50, specific area 42 m²/g) to which 5 by weight of Fe₂O₃, and 0.5% by weight of P₂O₅ have been applied as catalytically active material, which, after pelletisation and calcination has a BET area of 40.1 m²/g, with an average pore radius of 350 Å. Using this catalyst with an oxidation efficiency of 94%, a total sulphur recovery percentage of 99.7 is obtained. 

What is claimed is:
 1. A process for the recovery of sulphur from a hydrogen sulphide containing gas, which process comprises: i) oxidising part of the hydrogen sulphide in a gaseous stream with oxygen or an oxygen containing gas in an oxidation stage to sulphur dioxide; ii) reacting the product gas of this oxidation stage in at least two catalytic stages, in accordance with the Claus equation: 2 H₂S+SO₂→2 H₂O+3/n S_(n) iii) catalytically reducing SO₃ in the gas leaving the last of said at least two catalytic stages, wherein the catalytic reduction takes place in a hydrogenation catalyst placed downstream within the last Claus catalytic stage, thereby producing a gas mixture comprising less than 1.2 vol. % of H₂S; and iv) selectively oxidising H₂S in the gas obtained in step iii) to elemental sulphur in a dry oxidation bed and removing elemental sulphur from the product gas of this stage.
 2. Process according to claim 1, wherein the process conditions in steps i) and ii) are such, that the molar ratio of H₂S to SO₂ is between 10:1 and 1:2.
 3. A process as claimed in claim 2, wherein an H₂S concentration of 0.5-1.2% by volume is selected in the gas coming from the catalytic reduction stage iii).
 4. Process according to claim 2, wherein the hydrogenation catalyst is a compound containing a metal selected from group VI and VIII of the periodic table of elements.
 5. Process according to claim 2, wherein the hydrogenation is carried out at a temperature not exceeding 250° C.
 6. Process according to claim 2, wherein the hydrogenation catalyst has a cobalt-molybdenum compound.
 7. Process according to claim 2, wherein the hydrogenation catalyst has a cobalt-molybdenum compound.
 8. A process as claimed in claim 1, wherein an H₂S concentration of 0.5-1.2% by volume is selected in the gas coming from the catalytic reduction stage iii).
 9. Process according to claim 8, wherein the hydrogenation catalyst is a compound containing a metal selected from group VI and VIII of the periodic table of elements.
 10. Process according to claim 8, wherein the hydrogenation is carried out at a temperature not exceeding 250° C.
 11. Process according to claim 8, wherein the hydrogenation catalyst has a cobalt-molybdenum compound.
 12. Process according to claim 8, wherein the hydrogenation catalyst has a nickel-molybdenum compound.
 13. Process according to claim 1, wherein the hydrogenation catalyst is a compound containing a metal selected from group VI and VIII of the periodic table of elements.
 14. Process according to claim 13, wherein the hydrogenation is carried out at a temperature not exceeding 250° C.
 15. Process according to claim 1, wherein the hydrogenation is carried out at a temperature not exceeding 250° C.
 16. Process according to claim 1, wherein the hydrogenation catalyst has a cobalt-molybdenum compound.
 17. Process according to claim 1, wherein the hydrogenation catalyst has a nickel-molybdenum compound. 